1. Field of the Invention
The present invention relates to a picture coding method for coding picture signals at high efficiency.
2. Description of the Prior Art
The transmission speed of digitized picture signals has exceeded 100 Mbps, and it is difficult to transmit such signals directly by using the existing communication lines from the viewpoints of the transmission capacity and transmission cost. The picture coding is a technique for removing the redundancy of picture signals and reducing the transmission speed, and various methods and apparatuses had been proposed in the past. A representative example of orthogonal transform coding is described below.
FIG. 6(a) is an example of a conventional block coding method, showing a block diagram of a three-dimensional orthogonal transform coding apparatus, in which numeral 61 is a three-dimensional blocking circuit, 62 is a orthogonal transform circuit, and 63 is a quantizer.
FIGS. 6(b), (c) are explanatory diagrams of conventional three-dimensional block construction. In FIG. 6(b), numeral 1 denotes image data of a first frame, 2 is image data of a second frame, 3 is image data of a third frame, 11 is a two-dimensional block in the first frame, 12 is a two-dimensional block in the second frame, and 13 is a two-deminsional block in the third frame. In FIG. 6(c), the image data of each frame is drawn as two-dimensional plan view in the horizontal and time directions, and the frame in the time direction is illustrated as an example of up to three frames. In FIG. 6(c), numeral 71 denotes part of image data of the first frame, 72 is part of image data of the second frame, and 73 is part of image data of the third frame.
In thus composed three-dimensional orthogonal transform coding apparatus, the operation is described below.
In FIG. 6, the image input data s(h,v,t) (here h: picture element index in the horizontal direction, 1.ltoreq.h.ltoreq.M, v: picture element index in the vertical direction, 1.ltoreq.v.ltoreq.N, t: picture element index in the time direction, 1.ltoreq.t.ltoreq.T) entering the three-dimensional blocking circuit 61 is first divided into mXn two-dimensional blocks 71-1 in the first frame, and into 72-1 in the second frame 72, and so forth, and thus at the same two-dimensional position with frame, the two-dimensional blocks up to T-th frame are summed up thereafter in the time direction to be composed into mXnXT three-dimensional blocks.
The three-dimensional blocks are orthogonally transformed in the orthogonal transform circuit 62. The transform coefficient which is the output of the orthogonal transform circuit 62 is quantized in the quantizer 63 so that the quantizing bit allotment may be more in the coefficient of low sequence and be less in the coefficient of high sequence, and the quantized transform coefficient is later delivered as coding output. Generally, a natural picture is high in the correlation among the picture elements in the horizontal, vertical and time directions, and orthogonal transform coefficient is large in energy in low sequency and small in high sequency. Therefore, when quantizing, if small bits are assigned to the coefficient of high sequency, degradation is not obvious visually, and it is possible to compress while suppressing the picture quality degradation (for example, see chapter 12 of "Digital coding of waveform" by Jayant et al., published by Academic Press/Prentice-Hall).
In such conventional structure including three dimensions of the horizontal, vertical and time direction, in the case of a widely moving picture, the correction in the time direction is lowered and the efficiency is poor.